FIG. 7 shows a cross-sectional view of a known magnetic sensing element, which has a dual spin-valve structure.
This spin-valve magnetic sensing element includes a multilayer film 9, an electrode layer 1 disposed below the multilayer film 9, an electrode layer 10 disposed above the multilayer film 9, hard bias layers 11 disposed on both sides of a free magnetic layer 5, insulating layers 12 disposed below the hard bias layers 11, and insulating layers 13 disposed above the hard bias layers 11. The multilayer film 9 includes, from the bottom, an antiferromagnetic layer 2, a pinned magnetic layer 3, a nonmagnetic conductive layer 4, the free magnetic layer 5, a nonmagnetic conductive layer 6, a pinned magnetic layer 7, and an antiferromagnetic layer 8 , in that order.
The antiferromagnetic layers 2 and 8 are composed of PtMn. The pinned magnetic layers 3 and 7 and the free magnetic layer 5 are composed of CoFe. The nonmagnetic conductive layers 4 and 6 are composed of Cu. The hard bias layers 11 are composed of a hard magnetic material such as CoPt. The insulating layers 12 and 13 are composed of alumina. The electrode layers 1 and 10 are composed of a conductive material such as Cr.
In the magnetic sensing element shown in FIG. 7, the nonmagnetic conductive layer 4 and the pinned magnetic layer 3 are disposed below the free magnetic layer 5, and in addition, the nonmagnetic conductive layer 6 and the pinned magnetic layer 7 are disposed above the free magnetic layer 5. The dual spin-valve magnetic sensing element shown in FIG. 7 detects the recording magnetic field from a recording medium such as a hard disk.
The magnetic sensing element shown in FIG. 7 is a current-perpendicular-to-the-plane (CPP) magnetic sensing element wherein a current flows in a direction perpendicular to the surfaces of the layers of the multilayer film 9.
The magnetization directions of the pinned magnetic layers 3 and 7 are pinned in the Y direction in the figure. The magnetization of the free magnetic layer 5 forms a single magnetic domain in the track width direction (the X direction in the figure) by a longitudinal bias magnetic field from the hard bias layers 11. The magnetization of the free magnetic layer 5 rotates in response to an external magnetic field. As a result, the electrical resistance of the multilayer film 9 changes. This change in the electrical resistance is converted to a change in voltage or current, and the external magnetic field is thereby detected.
For example, Japanese Unexamined Patent Application Publication No. 2002-157711 discloses such a CPP dual spin-valve element.
According to the known CPP dual spin-valve element, each pair of the antiferromagnetic layer 2 and the antiferromagnetic layer 8, the pinned magnetic layer 3 and the pinned magnetic layer 7, and the nonmagnetic conductive layer 4 and the nonmagnetic conductive layer 6 are composed of the same material and have the same thickness. In other words, the multilayer film 9 has a symmetric structure above and below the free magnetic layer 5.
Recently it has been discovered that noise due to a spin transfer torque (STT) may be generated in the reproduction output of CPP magnetoresistive elements.
When a current flows in a direction perpendicular to the surfaces of layers of a multilayer film, including a free magnetic layer, a nonmagnetic conductive layer, and a pinned magnetic layer, a spin angular momentum of conduction electrons is transmitted to a spin angular momentum of magnetic materials forming the free magnetic layer and the pinned magnetic layer. As a result a spin transfer torque that causes the spin angular momentum of the free magnetic layer to be unstable is generated. When the spin angular momentum of the free magnetic layer becomes unstable, noise overlaps the reproduction output causing a decrease in the signal-to-noise ratio of the magnetic sensing element.
When conduction electrons flow in the direction from the free magnetic layer to the pinned magnetic layer, a torque is applied so that the magnetization direction of the free magnetic layer is antiparallel with respect to the magnetization direction of the pinned magnetic layer. On the other hand, when conduction electrons flow in the direction from the pinned magnetic layer to the free magnetic layer, a torque is applied so that the magnetization direction of the free magnetic layer is parallel with respect to the magnetization direction of the pinned magnetic layer.
Accordingly, when conduction electrons flow from the upper side to the lower side of the dual spin-valve magnetic sensing element shown in FIG. 7, a spin transfer torque applied to the free magnetic layer 5 by conduction electrons flowing from the pinned magnetic layer 7 to the free magnetic layer 5 and a spin transfer torque applied to the free magnetic layer 5 by conduction electrons flowing from the free magnetic layer 5 to the pinned magnetic layer 3 are canceled out with respect to each other. As a result, the noise is reduced.
However, in the known CPP dual spin-valve magnetic sensing element, which includes the multilayer film 9 having a symmetric structure above and below the free magnetic layer 5, the noise due to the spin transfer torque cannot be satisfactorily reduced.